Radio-frequency (RF) components, such as resonators and filters, based on microacoustics and thin-film technology are widely used in radio applications such as: mobile phones, wireless networks, satellite positioning, etc. Their advantages over their lumped-element counterparts include small size and mass-production capability. Two fundamental microacoustic technologies used for RF devices are surface acoustic wave (SAW) and bulk acoustic wave (BAW) technologies.
In this section, existing filter technologies are briefly introduced to provide background for the current invention.
Surface Acoustic Wave Devices
A schematic picture of a SAW device is shown in FIG. 1(a). Interdigital transducers (IDTs) 2, 3 (comb-like structures of thin-film metal strips) are patterned on a piezoelectric substrate 1. The piezoelectric substrate is, for example, quartz, LiNbO3 or LiTaO3. The IDTs are used to transform the electric input signal Vin into an acoustic wave via the piezoelectric effect, as well as to pick up the acoustic signal at the output port and transform it back to an electrical form. The operation frequency of a SAW device depends on the velocity of the acoustic wave and the dimensions of the IDT electrodes:
  f  ∝      v          2      ⁢                          ⁢      p      where f is the frequency, p is the period of the IDT, see FIG. 1(b), and v is the velocity of the surface wave. Therefore, higher operation frequencies require smaller p if the velocity is kept constant.
SAW transducers are typically periodic, although the period may be more complex than that presented in FIG. 1.
Bulk Acoustic Wave Devices
In a BAW device, acoustic vibration inside a piezoelectric wafer or a thin film is used to process the electrical input signal. The piezoelectric material used for the thin film in the devices typically belongs to the 6 mm symmetry group, e.g. ZnO, AlN and CdS. Other piezoelectric materials can be used as well, such as quartz, LiNbO3, LiTaO3, etc. A schematic cross-section of a solidly-mounted BAW resonator (SMR) is presented in FIG. 2(a) and of a self-supported (membrane type) resonator in FIG. 2(b). In an SMR, an acoustic Bragg mirror composed of alternating high and low acoustic impedance (Z) material layers serves to isolate the vibration in the piezoelectric thin film from the substrate and to prevent acoustic leakage. In the membrane device the same is accomplished by fabricating the resonator on a self-standing membrane.
The area of a BAW resonator is typically determined by the static capacitance needed to match the device to the system impedance. Filters can be constructed of resonators by connecting resonators electrically. A common example is a ladder filter, in which resonators are connected in T- or Pi-sections (FIG. 3). Designing the resonance frequencies appropriately, one can achieve a passband response. Increasing the number of sections helps to widen the passband. The out-of-band signal suppression is determined by the capacitances of the resonator structure and is typically on the order of ˜25 dB. The in-band losses are mainly determined by the Q-values of the resonators.
Vibration Modes and Dispersion Types
In the piezoelectric layer of an acoustic resonator, different bulk acoustic vibration modes arise as the excitation frequency f is swept. In BAW devices, the propagation direction of the bulk wave is typically along the thickness axis (z axis). Particle displacement is either perpendicular to the propagation direction (shear wave) or parallel to the propagation direction (longitudinal wave). Bulk modes are characterized by the number of half-wavelengths of the bulk wavelength λz that can fit into the thickness of the resonator structure (piezoelectric layer and electrodes). In addition, the bulk modes can propagate in the lateral (perpendicular to z-axis) direction as plate waves with lateral wavelength λ∥. This is illustrated in FIG. 4a for two bulk modes (longitudinal and shear). In a finite-sized resonator, plate waves reflecting from resonator edges can cause laterally standing waves and consequently lateral resonance modes.
Acoustic properties of a BAW resonator can be described with dispersion curves, in which the lateral wave number k∥ of the vibration is presented as a function of frequency. FIG. 4b shows an example of dispersion properties in a BAW resonator. Dispersion curves of the electroded (active) region are plotted with a solid line and those of the non-electroded (outside) region with dashed line. The first-order longitudinal (thickness extensional, TE1) vibration mode, in which the thickness of the piezoelectric layer contains approximately half a wavelength of the bulk vibration, and the second-order thickness shear (TS2) mode, in which the bulk vibration is perpendicular to the thickness direction and one acoustic wavelength is contained in the piezoelectric layer thickness, are denoted in the figure. This type of dispersion, in which the TE1 mode has increasing k∥ with increasing frequency, is called Type 1. Type 1 materials include, e.g. ZnO. Aluminum nitride is inherently Type 2 (in FIG. 4b, TE1 mode is the lower dispersion curve, and TS2 mode is the upper dispersion curve), but with an appropriate design of the acoustic Bragg reflector, the resonator structure's dispersion can be tailored to be of Type 1.
In FIG. 4b, positive values of k∥ denote real wave number (propagating wave) and negative values correspond to imaginary wave number (evanescent wave). For a resonance to arise, the acoustic energy must be trapped inside the resonator structure. In the thickness direction, isolation from the substrate (mirror or air gap) ensures the energy trapping. In the lateral direction, there should be an evanescent wave outside the resonator region for energy trapping. Energy trapping is possible between frequencies fo1 and fo2. Frequency range for which energy trapping occurs for the TE1 mode, fo2-fa, is shaded in FIG. 4b. Energy trapping is easier to realize in Type 1 dispersion. Therefore, when using AlN as the piezoelectric material, the mirror is usually designed so that it converts the dispersion into Type 1. This limits somewhat the design of the acoustic mirror.
As the lateral wave number k∥ increases on a dispersion curve (lateral wavelength decreases), lateral standing wave resonances (plate modes) appear in the resonator structure. For a plate mode to arise, the width of the resonator W must equal an integer number of half wavelengths of the plate mode:
  W  =      N    ⁢                  λ        m            2      for the mode m with wavenumber km=2π/λm.Acoustical Coupling in BAW Devices
A filter can be made by electrically connecting one-port resonators to form a ladder or a lattice filter. Another possibility is to arrange mechanical (acoustic) coupling between resonators by placing them close enough to each other for the acoustic wave to couple from one resonator to another. Such devices are called coupled resonator filters (CRF).
In BAW devices, vertical acoustic coupling between stacked piezoelectric layers is used in stacked crystal filters (SCF, see FIG. 5(a)) and vertically coupled CRFs (FIG. 5 (b)). In an SCF, two piezoelectric layers are separated by an intermediate electrode. In a vertically coupled CRF, coupling layers are used to modify the coupling strength between the piezo layers. The CRF can be fabricated either using the SMR or the air-gap technology.
A thin-film vertically coupled CRF has been shown to give a relatively wide-band frequency response (80 MHz at 1850 MHz center frequency, or 4.3% of center frequency, FIG. 8(a) from G. G. Fattinger, J. Kaitila, R. Aigner and W. Nessler, “Single-to-balanced Filters for Mobile Phones using coupled Resonator BAW Technology”, Proc. IEEE Ultrasonics Symposium, 2004, pp. 416-419) with the capability of unbalanced-to-balanced (balun) conversion. The disadvantage of the vertically coupled CRFs is the need for a large number of layers and their sensitivity to the thickness of the piezolayers. This makes the fabrication process difficult and consequently expensive.
Lateral acoustical coupling in BAW (LBAW) can be realized with 2 or more narrow electrodes placed on the piezoelectric layer 1 (FIG. 6) on a thin layer structure 4, in such a way that the acoustic vibration can couple in the lateral direction from one electrode to another. Electrical input signal in Port 1, 5 is transformed into mechanical vibration via the piezoelectric effect. This vibration couples mechanically across the gap to Port 2, 6 and creates an output electrical signal. Electrodes in this example are interdigital (comb-like). Coupling strength is determined by the acoustic properties of the structure and by the gap between the electrodes.
Bandpass frequency response is formed by two laterally standing resonance modes arising in the LBAW structure, as illustrated in FIG. 7 for a two-electrode structure. In the even mode resonance, both electrodes vibrate in-phase, whereas in the odd mode resonance their phases are opposite. For a Type 1 resonator, the even mode, having a longer wavelength, is lower in the frequency than the shorter-wavelength odd mode. The frequency difference between the modes determines the achievable bandwidth of the filter, and depends on the acoustic properties of the structure and on the electrode dimensions.
Vertical CRF's have disadvantages as they are difficult and costly to fabricate, i.e. they require several layers and are sensitive to piezoelectric films' thickness.
LBAW is therefore advantageous because it has a simple fabrication process and the operation frequency is mainly determined by the piezolayer thickness, though to a lesser extent by electrode geometry. So far however, the obtained bandwidth has been too narrow, i.e. 2%-3% of the center frequency.
One problem with LBAW filters is the gently sloping edges of the pass band, which can limit the area of application of the components. By steepening the sloping edges, the competitiveness of LBAW filters would improve significantly.